This invention relates to integrated circuit processing involving ion implantation, and more particularly to a method of producing ion implants in regions of a semiconductor body whose edges are self-aligned with a cut or opening in a stratum of material that lies above the implanted regions.
In fabricating semiconductor integrated circuits, it is sometimes required to produce ion deposits in spaced regions of the semiconductor body so that the space between the ion deposits is in alignment with an opening in a layer of insulating material lying above the ion deposits. In metal-oxide-semiconductor (MOS) circuits, for example, a thick field oxide layer is formed on the surface of the semiconductor body. In the finished device the field oxide serves as an insulating support for conductive members as well as a means for electrically isolating one active transistor region from the other that lie in voids formed in the thick field oxide layer. During device processing the field oxide layer serves as a mask for selective chemical etching as well as selective impurity doping. Beneath the field oxide in the semiconductor body lie ion deposits or channel stops of sufficient ion concentration to prevent undesired transistor action anywhere outside the active transistor regions.
A typical procedure followed in MOS processing is to introduce the ion deposits for the channel stops into the semiconductor body before the field oxide is thermally grown on the surface of the semiconductor body. There are several drawbacks that result from this procedure. For one thing, there is a tendency for some of the ion deposits to diffuse into the active transistor regions as a result of thermal activation, thereby narrowing the active regions.
Another effect that contributes to narrowing the active regions is the so-called "bird-beak effect" in the selective oxidation process. When the field oxide is thermally grown, it grows both above the surface as well as into the body of the semiconductor wafer and beneath the oxidation masking material, such as silicon nitride. As it grows beneath the oxidation mask, both laterally and depthwise, the field oxide lifts the oxidation mask, so that when viewed in cross section, the shape of the field oxide layer resembles the beak of a bird, being thick at the unmasked regions and tapering to a point somewhat inwardly beneath the oxidation mask. The "bird-beak effect" also contributes to narrowing the active regions because it extends the field oxide layer into a part of the masked region, thus cutting down the width of the field-oxide-free region the oxidation mask was designed to produce.
Besides narrowing the width of the transistor active regions, the "bird-beak effect" also narrows the width of strip conductors that will be formed by heavy dosage of dopant, implanted or diffused in regions of the wafers that are devoid of field oxide, thereby reducing the conductance of the strip conductors.
According to another technique for forming the active regions in the semiconductor surface, a hole is etched through the field oxide layer and the bare silicon surface is etched in the active region to remove ion deposits that were previously introduced to form the channel stop regions. The silicon etch produces undercutting of the silicon beneath the field oxide. The undercutting is detrimental in that the surface irregularities introduce light reflection problems during later photolithographic processing steps. The increased step height from the active region to the top of the thick field oxide that results from the silicon etch also aggravates the step coverage problem for metallic interconnects in and out of the active regions.
There is a great need to eliminate or at least minimize the undesirable effects that result from prior art processing.